"I wouldn't go so far as to call it an injury, really," Matsuzaka said through interpreter Masa Hoshino. "I think, going into the offseason last year, I really didn't take a break, and I got right back into a training program. So I think it was just a little bit of fatigue that built up over that time period."
Though it's just a nagging ailment, Matsuzaka admits that it's the type of thing he might have neglected to mention to the Red Sox a year ago. And thus, it's the type of thing that could have snowballed.
"I think so," Matsuzaka said. "I think that if a similar situation had happened in the past -- I feel like I have a pretty high pain tolerance, so if I can tolerate the pain and still somehow play through and be able to do what I had to do, I think I would have taken a lot of that on myself.
"But I definitely don't want to make the same mistakes that were made last year, so right now, I think it's important for me to talk about my condition and share that with the team in as much detail as possible."
The problem last year, Matsuzaka has only recently confided, is that his groin started to bother him at the World Baseball Classic and he never said anything about it.
"As for the groin injury, being able to move my lower body effectively is really my lifeline in how I pitch," Matsuzaka said. "I think I was able to get away with it in the [Classic], but once you start throwing in the big leagues, it's a different stage, it's a different level of competition.
"I just wasn't able to get away with it. That was really the learning experience for me. It was a difficult season, and I'm not going to let that go to waste. By going through that experience, I think I'm going to be able to come out on the other side a lot stronger, and I think I'm going to be able to transfer a lot of that into this upcoming season."
One thing is certain -- Matsuzaka clearly looks stronger and trimmer. That process started during a grueling rehab last summer in Fort Myers. It continued over the winter at the Athletes' Performance Institute in Arizona.
Now halfway through a six-year, $52 million contract the Red Sox signed him to, Matsuzaka doesn't see any reason why his best years aren't ahead of him.
"Going into my fourth year here, I think that this was my best offseason in terms of preparation," Matsuzaka said. "[Aside from] this present situation, it was a good offseason for preparation. I can't say for sure until I start throwing the ball, but my expectations for this season are very high."
His exuberance for 2010 is the reason why the back ailment, however minor, is an annoyance.
"I wanted to go into Day 1 of Spring Training really ready to get after it and being prepared to throw a lot of pitches," Matsuzaka said. "But right now, this is something I have to deal with, and I have to bear with it for the time being. But for right now, it's a little stressful."
But the Red Sox are pleased with the way he is dealing with it.
"I think there's better communication," said Red Sox manager Terry Francona. "Good players get to be good because they want to compete. If you didn't go out there when you weren't 100 percent, no one would ever be on the field. [But] the more you communicate, the better, for sure."
When will Matsuzaka start throwing again? That will be determined by what the doctors say after his physical on Friday.
But Matsuzaka seems more sure of himself that at any other period of his time in Boston. A lot of it was natural assimilation that happens over time.
"I think I'm able to approach the season and come to work here just as I was able to do in Japan, so I think that I've definitely gotten used to things over here," Matsuzaka said. "With each passing year, as I've become more and more accustomed to how things work, I think I look forward to the season just that much more every year."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.